RESUMEN
van der Waals heterostructures composed of two-dimensional (2D) transition metal dichalcogenides and vdW magnetic materials offer an intriguing platform to functionalize valley and excitonic properties in nonmagnetic TMDs. Here, we report magneto photoluminescence (PL) investigations of monolayer (ML) MoSe2 on the layered A-type antiferromagnetic (AFM) semiconductor CrSBr under different magnetic field orientations. Our results reveal a clear influence of the CrSBr magnetic order on the optical properties of MoSe2, such as an anomalous linear-polarization dependence, changes of the exciton/trion energies, a magnetic-field dependence of the PL intensities, and a valley g-factor with signatures of an asymmetric magnetic proximity interaction. Furthermore, first-principles calculations suggest that MoSe2/CrSBr forms a broken-gap (type-III) band alignment, facilitating charge transfer processes. The work establishes that antiferromagnetic-nonmagnetic interfaces can be used to control the valley and excitonic properties of TMDs, relevant for the development of opto-spintronics devices.
RESUMEN
Coherence length (Lc) of the Raman scattering process in graphene as a function of Fermi energy is obtained with spatially coherent tip-enhanced Raman spectroscopy. Lc decreases when the Fermi energy is moved into the neutrality point, consistent with the concept of the Kohn anomaly within a ballistic transport regime. Since the Raman scattering involves electrons and phonons, the observed results can be rationalized either as due to unusually large variation of the longitudinal optical phonon group velocity vg, reaching twice the value for the longitudinal acoustic phonon, or due to changes in the electron energy uncertainty, both properties being important for optical and transport phenomena that might not be observable by any other technique.
RESUMEN
Near field scanning Microwave Impedance Microscopy can resolve structures as small as 1 nm using radiation with wavelengths of 0.1 m. Combining liquid immersion microscopy concepts with exquisite force control exerted on nanoscale water menisci, concentration of electromagnetic fields in nanometer-size regions was achieved. As a test material we use twisted bilayer graphene, because it provides a sample where the modulation of the moiré superstructure pattern can be systematically tuned from Ångstroms up to tens of nanometers. Here we demonstrate that a probe-to-pattern resolution of 108 can be obtained by analyzing and adjusting the tip-sample distance influence on the dynamics of water meniscus formation and stability.
RESUMEN
Twisted bilayer graphene is created by slightly rotating the two crystal networks in bilayer graphene with respect to each other. For small twist angles, the material undergoes a self-organized lattice reconstruction, leading to the formation of a periodically repeated domain1-3. The resulting superlattice modulates the vibrational3,4 and electronic5,6 structures within the material, leading to changes in the behaviour of electron-phonon coupling7,8 and to the observation of strong correlations and superconductivity9. However, accessing these modulations and understanding the related effects are challenging, because the modulations are too small for experimental techniques to accurately resolve the relevant energy levels and too large for theoretical models to properly describe the localized effects. Here we report hyperspectral optical images, generated by a nano-Raman spectroscope10, of the crystal superlattice in reconstructed (low-angle) twisted bilayer graphene. Observations of the crystallographic structure with visible light are made possible by the nano-Raman technique, which reveals the localization of lattice dynamics, with the presence of strain solitons and topological points1 causing detectable spectral variations. The results are rationalized by an atomistic model that enables evaluation of the local density of the electronic and vibrational states of the superlattice. This evaluation highlights the relevance of solitons and topological points for the vibrational and electronic properties of the structures, particularly for small twist angles. Our results are an important step towards understanding phonon-related effects at atomic and nanometric scales, such as Jahn-Teller effects11 and electronic Cooper pairing12-14, and may help to improve device characterization15 in the context of the rapidly developing field of twistronics16.
RESUMEN
Determining the role of defects in materials can be an important task both for the fundamental understanding of their influence on material properties and for future applications. In this work, we studied the influence of defects on the second harmonic generation (SHG) in hexagonal boron nitride (h-BN). We characterized the sample by photoluminescence imaging and spectroscopy, showing strong and sharp photoluminescence emission at visible range from h-BN flakes due to single defect states. By doing second harmonic imaging, we found strong emission from the h-BN flakes that correlates spatially with the photoluminescence imaging. By doing polarization-resolved SHG, we found deviations from the expected polarization pattern in pristine h-BN samples. We also characterized the nonlinear optical susceptibility of h-BN with defects with a value of one order of magnitude larger than for pristine h-BN, which highlights the role of defects in the efficiency of SHG. Therefore defect engineering could be used as a potential tool for nonlinear optical signal enhancement.
RESUMEN
Light-matter interaction in two-dimensional photonic or phononic materials allows for the confinement and manipulation of free-space radiation at sub-wavelength scales. Most notably, the van der Waals heterostructure composed of graphene (G) and hexagonal boron nitride (hBN) provides for gate-tunable hybrid hyperbolic plasmon phonon-polaritons (HP3). Here, we present the anisotropic flow control and gate-voltage modulation of HP3 modes in G-hBN on an air-Au microstructured substrate. Using broadband infrared synchrotron radiation coupled to a scattering-type near-field optical microscope, we launch HP3 waves in both hBN Reststrahlen bands and observe directional propagation across in-plane heterointerfaces created at the air-Au junction. The HP3 hybridization is modulated by varying the gate voltage between graphene and Au. This modifies the coupling of continuum graphene plasmons with the discrete hBN hyperbolic phonon polaritons, which is described by an extended Fano model. This work represents the first demonstration of the control of polariton propagation, introducing a theoretical approach to describe the breaking of the reflection and transmission symmetry for HP3 modes. Our findings augment the degree of control of polaritons in G-hBN and related hyperbolic metamaterial nanostructures, bringing new opportunities for on-chip nano-optics communication and computing.
RESUMEN
In this work we probe the third-order nonlinear optical property of graphene and hexagonal boron nitride and their heterostructure by the use of coherent anti-Stokes Raman spectroscopy. When the energy difference of the two input fields matches the phonon energy, the anti-Stokes emission intensity is enhanced in h-BN, as usually expected, while for graphene an anomalous decrease is observed. This behavior can be understood in terms of a coupling between the electronic continuum and a discrete phonon state. We have also measured a graphene/h-BN heterostructure and demonstrate that the anomalous effect in graphene dominates the heterostructure nonlinear optical response.
RESUMEN
We report on magnetotransport studies of dual-gated, Bernal-stacked trilayer graphene (TLG) encapsulated in boron nitride crystals. We observe a quantum Hall effect staircase which indicates a complete lifting of the 12-fold degeneracy of the zeroth Landau level. As a function of perpendicular electric field, our data exhibit a sequence of phase transitions between all integer quantum Hall states in the filling factor interval -8<ν<0. We develop a theoretical model and argue that, in contrast to monolayer and bilayer graphene, the observed Landau level splittings and quantum Hall phase transitions can be understood within a single-particle picture, but imply the presence of a charge density imbalance between the inner and outer layers of TLG, even at charge neutrality and zero transverse electric field. Our results indicate the importance of a previously unaccounted band structure parameter which, together with a more accurate estimate of the other tight-binding parameters, results in a significantly improved determination of the electronic and Landau level structure of TLG.